Technical Notes &
Applications

This
section of our web site is a tutorial style technical discussion on the application and
specification of receiver multicouplers, as well as on support devices such as
pre-amplifiers and receive filters and related components.
The material is mainly from our own R&D, but also from practical field test and
experience with receiver multicouplers for a variety of radio monitoring sites.

We expect to include new and useful material for this
page on a continuous basis, therefore, it always will stay "Under Construction".
If you have a technical or an application question not fully covered here, please
email
or give us a call, we will be happy to talk to you.

The most fundamental use of a receiver multicoupler is to enable several
radio receivers to utilize a single antenna system without interaction that may degrade
each receiver's performance. Many variations in the design concept do exist as well as
circuit options, however, signal splitting and the distribution of an RF signal is the
basic need to be accomplished.

Receiver multicouplers may
be used for all levels of signal monitoring, from VHF/UHF scanner radios used to monitor
agencies, HF receivers to monitor shortwave broadcasts or utility stations, etc. The
implementation of a monitoring system using multicouplers, from HF to VHF/UHF, may
range from the level of the serious hobbyist to surveillance professionals engaged in the
business of signals intercept and communications monitoring. There are many benefits in
using receiver multicouplers but the most obvious one is, of course, that only one antenna
system is required to feed several receivers. The inherent technical layout of the
monitoring system is further simplified in that only one feed-line is needed, and if a
pre-amplifier or a lightning protection device is used, here again, only one is needed to
feed banks of monitoring receivers.

Receiver multicouplers are available in two basic categories, the Passive
and Active versions. Both will perform the basic task of splitting an RF
signal and feeding several radios without degrading performance of each receiver in the
system. The primary difference between a Passive and an Active
unit is in how signal losses due to signal splitting is corrected for. That is, normal
signal losses always occur in a passive coupler - 3, 6, 12 dB, etc. depending on the
number of output ports. Active couplers have circuit designs that correct for these losses
in addition to High or Low pass filters for bandwidth control. Therefore, as an example, a
4-port Active multicoupler has no loss of signal from input to either one of its four
outputs. The number of outputs (referred to as "ports") may range from 2
up to 16 or 32, however, standard stock units manufactured by Stridsberg is limited to 2,
4 and 8-port configurations.

In the "Technical Notes" section that follows, we will
discuss specifications and common applications of the Receiver Multicoupler. It will be
helpful in the selection of a specific unit and demonstrate what a receiver multicoupler
can or can not do.We know that each monitoring site has different technical and operational needs,
from a few scanners at a home based monitoring post, to, 32 receiver SIGINT platforms (or
something in-between). However, there is a common need, antenna signals must be routed to
each receiver in such a way as not to compromise the electrical performance of each radio.
To meet that need, Stridsberg Engineering manufactures a standard line of Active and
Passive multicouplers with specifications and reliabilty that satisfy the requirements of
any level of user application.

For the majority of
of users, specifications of importance are port-to-port Isolation and the
coupler's signal loss/gain characteristics. And in the active multicoupler, front-end
dynamic specs such as P1dB and 3IP are of value to know. However, the most important
specification of a receiver multicoupler (passive or active) is the Port-to-Port
Isolation. Isolation is measured in dB and is an indication of how well an output
is (signal wise) separated (isolated) from the other output(s). The issue of isolation is
very important when several radio receivers are connected to the multicoupler. Most radio
receivers will emit RF signals back out (reverse) the antenna. These signals are,
unfortunately, a natural circumstance from local oscillators, mixers, PLL synthesizers,
etc. Depending on receiver circuit design these "outgoing" signals may be
substantial - being strong enough in some cases to be detected by other nearby receivers.
Therefore, when using several receivers in a monitoring set-up using a multicoupler, good
isolation between ports is important. It ensures that each individual radio is operating
without interference from other radios using the coupler. Isolation shall be a minimum of
20 dB for effective use -"the more the better" applies here.

Figure 1 above shows how a standard multi-receiver
site may be configured. In this case a 4-port unit is used to connect 4 receivers to one
antenna. With good isolation between the ports, these receivers will work as they had
independent and separate antennas connected to them, and, without interference between
radios - the main benefit of the multicoupler.

In figure 2., one method of isolation
measurement is shown. This set-up is the preferred one when swept measurements are
necessary to confirm isolation over a broad frequency range, example, 25 MHz to 1000 MHz
range. All multicouplers manufactured by Stridsberg Engineering are tested this way before
release to customer.

There are, of course, several other
electrical and mechanical specification that will determine the performance of a receiver
multicoupler, both passive and active models. Among them is signal gain and or loss,
signal phase shift (input/output), strong signal handling, power supply requirement,
enclosure shielding, etc. Contact Stridsberg Engineering for information on these
specifications as it applies to a specific model, or, if they are important or relevant to
your specific application.

As mentioned above, in most radio receiving applications using multicouplers, the
isolation (rated in dB) is by far the primary parameter of outmost importance. Good
isolation will enable multiple receivers to work to its sensitivity specifications, and in
the case of scanning radios, not "lock up" on signals emitted from other radios
on the same antenna.

Passive vs. Active? In the selection process of a receiver
multicoupler the question of whether or not a passive unit can do the job, or, if an
active coupler must be used, will have to be answered. In most cases it is relatively easy
and straight forward to determine the best choice for a given monitoring site as it is,
generally, a question of RF signal strength (or anticipated signal strength) of the
transmitters being monitored. For example, if strong HF broadcast stations are primarily
monitored, such as BBC, VOA and other international radio stations with substantial signal
strength, a passive multicoupler will in most cases do a fine job. Specifically if a large
antenna system is used, the passive coupler may be a good choice. If, on the other hand,
weaker signals with varying strengths such as those that may be received from clandestine
radio transmitters, or from utility stations, is the primary goal of the monitoring
effort, an active receiver multicoupler is the better choice.Stridsberg Engineering offers passive units for both HF and
VHF/UHF with up to 4-ports. The larger 8-port multicouplers are available only as active
units due to the excessive signal losses that would normally occur in a
passive unit with that number of output ports (-13 to -14dB loss, input to any output
port).

On VHF and UHF frequencies, the active multicoupler is
the best choice in most cases. Here, the signal strengths of received signals are,
generally, lower than what would be encountered on the HF frequencies. Also, increased
losses in the antenna feed-line as the frequency is increased produces lower signal levels
at the radio. Specifically, from about 500 MHz and up. Depending on the coaxial cable
used, these losses may be substantial. Losses ahead of a radio's front-end (such as in the
coax) will directly contribute to system noise figure, therefore, selection of an active
multicoupler will not add to those losses as the passive unit would.

Active receiver multicouplers for VHF/UHF monitoring such as the MCA202M,
MCA204M and MCA208M are capable of delivering RF signals to any of
the output ports without loss. The frequency coverage of these units range from 25
MHz to 1 GHz, and beyond with slightly relaxed specifications. In fact, the active
couplers are useable up to about 1.2 GHz. They are designed with internal amplifiers that
corrects for losses, in addition to high-pass filters to shape and control the band-pass
on these relatively broadband units. For a monitoring site with several VHF/UHF radios,
the active coupler will always give better performance than a passive unit.

For HF monitoring, models MCA102M, MCA104M
and MCA108M are active receiver multicoupler suitable for continuos
service from about 100 kHz to 50 MHz. The high end of the frequency coverage (50 MHz) is
set by a low-pass filter, thus, protecting the internal amplifier from strong out of band
VHF/UHF signals, such as FM radio and TV signals. Although the internal amplifier does not
have the benefit of having an AGC signal applied to it at the onset of
strong signals, such as a receiver front-end, they are nevertheless designed
very robust. Generally, standard models of the active couplers are specified
with front-end dynamic numbers of P1dB of +17 dBm and P3IP of + 31 dBm.

All active multicouplers requires +12 VDC/100 mA
(nominal) to operate. Because of internal voltage regulation and filtering, this input DC
voltage may be between +10 volt and +17 volts. On our standard models, DC power is applied
via a common coaxial style power jack (5.5mm/2.1mm) with the center pin
positive.

For the North American market, a wall adapter
(110VAC/12VDC) is supplied as an accessory. For other countries, using AC power standards
of 220 Volts, a DC power plug is supplied for the DC connection, this plug must wired by
customer. On request, we can supply this plug wired with about 6 feet of cable for
connection to customer +12 VDC power source.

The
MCA series of multicouplers are designed to be used in mobile applications as well
(surveillance vans, aircraft and shipboard SIGINT/COMINT platforms). If +12 VDC power is
available, the multicoupler may be used with the supplied DC plug.For other special DC
power requirements, please give us a call.

Page Under Construction

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